Illumination system and method for a high definition light microscope

ABSTRACT

An illumination system and method for increasing resolution, sharpness, depth of field, and perception of depth for a transmitted light microscope including a condenser lens having an optical axis, objective lens having an optical axis, and an eye piece wherein a light beam path shift enables two or more separate light beams to be directed onto the condenser along paths that are not coincident with the condenser lens optical axis and that produce beam exit paths from the condenser lens which are at maximum oblique angles relative to the objective lens axis which are within the objective lens aperture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to illumination systems for transmitted lightmicroscopes utilizing condenser lens means and more particularly to suchsystems that utilize one or more illuminating light beams whose axes aredirected at an oblique angle relative to the optical axis of themicroscope objective.

2. The Prior Art

The use with microscopes of what is commonly refereed to as "obliquelight" was of interest towards the end of the last century and thebeginning of this, but the many devices designed for that purpose,although ingenious in some cases, have failed to survive. See TheIntelligent Use of the Microscope, Oliver, C. W., Chemical PublishingCo., 1953.

Oliver carefully limits his meaning of "oblique light" to the "use of anarrow cone or beam of rays directed upon the object [specimen] from anydirection other than the optical axis provided that it enters the objectglass." Ibid. at 94. In this way he excludes from his discussion thosesystems that use rays directed onto a specimen from a direction otherthan the optical axis but which do not enter the object glass as well assystems where the light does not enter the objective lens at an angle(such as systems that merely tilt the specimen stage). Illuminationprovided by systems in which the primary beam does not enter theobjective is generally known and commonly refereed to as "dark field"illumination as more fully discussed in Photomicrography a ComprehensiveTreatise, Loveland, R. P., John Weily & Sons, Chapter 12. Although thepresent invention utilizes true oblique lighting as that term is used byOliver, and is thereby clearly distinguishable from "dark field"systems, a brief description of "bright field" and "dark field"illumination will help to differentiate and more fully highlight theattributes of the present invention.

Illumination systems that direct rays onto a specimen along the opticalaxis create "bright field" illumination, so named because the rayspassing through the field surrounding the specimen and entering themicroscope objective are unimpeded and thus bright compared to the raysattenuated by passing through the specimen. In a "dark field" system,the relative brightness is reversed by directing only light rays ontothe specimen field which are angled relative to the optical axis anddirected to fall outside the objective aperture. All of the lightpassing through the specimen field surrounding the specimen is unimpededand thus does not enter and is therefore not "seen" by the objective.Some of the light directed onto the specimen will be scattered, however,into secondary light rays, some of which will enter the objective (andbe "seen"). Thus, the object appears brighter than the surrounding darkfield. Such a system is described in U.S. Pat. No. 4,896,966.

The prior art contains a number of systems that combine "bright field"and "dark field" illumination for use both together and selectively, asillustrated in U. K. Pat. No. 87,230, and U.S. Pat. No. 4,601,551. Inall of these systems the primary illuminating light is either alignedwith the optical axis or angled to fall outside of the objectiveaperture.

The invention of U.S. Pat. No. 3,876,283, teaches the use of a systemwhich uses true oblique lighting, by use of a prism located on theoptical axis of a microscope condenser to laterally off-set an axialillumination beam to a path separate from the optical axis so as todirect the beam onto an off center location on the condenser lens. Whena light beam parallel to the optical axis enters an off center locationon a condenser lens, the beam will exit the lens at an angle to theoptical axis. The degree of the angle is a function of the displacementof the beam from the center of the lens. When, as in patent '283, theangle is within the objective aperture, the system produces true obliquelighting as defined by Oliver the light is "seen" by the objective). Inorder to achieve the maximum oblique angle for the beam it must exit thecondenser lens at or very near its periphery at an angle that is justwithin the objective aperture. While the teachings of patent '283 makethis possible (by adding a wedge shaped prism to the plano prism shown),each different condenser and objective combination will require adifferent pair of prisms to achieve a maximum angle. Otherwise,depending on the characteristics of the objective lens and condenserlens being used, it may be necessary with the system of patent '283 todirect the laterally off-set beam onto the condenser lens at a locationinwardly of its periphery in order to have the resultant exit anglewithin the objective aperture. In such cases the maximum possibleoblique angle will not be realized and, as will be explained below, themaximum resolution power of the system will not be achieved.

In patent '283, the location of the illuminating beam (between 15 and17) and beam path shifting means 23 (prism) on the optical axis limitsthe system by permitting the use of only a single illumination beam.

The references cited above are typical of the prior art in that theyfail to recognize the real potential of oblique lighting to enhanceresolution. Patent '283, in fact, does not acknowledge the resolutionenhancing potential of oblique light but instead cites as a reason forits use the casting of shadows to highlight uneven areas of thespecimen. It is not, therefore, necessarily an object or desideratum ofpatent '283 to provide a maximum oblique angle (for example, too muchshadowing might obscure details). But, one of the requirements ofrealizing the full potential of oblique lighting to dramatically enhanceresolution is that the angle of the oblique light be maximized. For asingle beam system, maximum resolution is achieved for a given condenserlens/objective lens combination by having the illumination beam exit thecondenser lens' periphery so that the light illuminating the object isat a maximum oblique angle and still within the objective aperture. Bymaking it possible to adjust the angle at which the beam exits thecondenser lens independently of the location where it exits, the angleof the light (relative to the optical axis of the objective lens means)can be fully maximized. Likewise, by being able to adjust the locationwhere the beam exits the condenser independently of the angle at whichit exits, any condenser can be used to its fullest potential. With theability to so adjust the angle and location of the beam exiting thecondenser lens, a large condenser lens (high numerical aperture) can beused to achieve maximum oblique lighting for most objective lenses.

The present invention teaches that the essential requirement forrealizing the maximum potential of true oblique lighting is the abilityto direct two or more separate and distinct light beams onto thecondenser wherein each beam is at the maximum angle to the objectiveaxis that permits the illumination to enter the objective. This, ofphysical necessity, requires that the beam shifting means be located offthe optical axis of the condenser. In addition, the present inventionovercomes the anistropy that is found in prior art oblique illuminatingsystems.

In addition,the present invention teaches a real time, 3-D system usingmultiple beams which goes far beyond what can be achieved with a singlebeam, such as that described in U.S. Pat. No. 4,072,967. Patent '967teaches how to achieve a 3-D image using a microscope with a singlecondenser lens and a single objective lens, by placing complimentaryfilters across the left and right halves of the condenser lens andplacing a complementary filter set in the binocular eyepieces. With thistype of system the degree of parallax is fixed. Furthermore, there isvery little disparity in parallax between the left and right images,especially at the center of the image field. In contrast, with thepresent invention the left and right images are independently controlledand the degree of parallax between them can be easily adjusted to matchthe type of objective being employed and the type of specimen beingviewed. In addition, there is another and possibly even more importantadvantage with the present invention, which is the ability to achieve agreater depth of field without loss of resolution, as is more fullyexplained below. This is a critical prerequisite for producing a sharp3-D image.

SUMMARY OF THE INVENTION

The present invention resides in the illumination system for atransmitted light microscope characterized by condenser lens means(which can be comprised of several lenses) and an objective lens means(also comprised of several lenses). The object or specimen to beilluminated is located between the condenser and the objective.

The diffraction theory of microscopic vision teaches that when examiningwith transmitted light an object having very closely spaced structuraldetails such as the markings of the diatom Amphipleura pellucida, theresolution and sharpness of the tiny lines in the image depend upon thenumber of orders of diffraction wavelets that are collected by theobjective lens and used to form the image. The number and arrangement ofthe diffraction orders depends upon the lines in the object and thewavelength of light being used and other factors, the finer the detailon the object the greater the angular separation between the orders. Themore successive orders of wavelets recombined with the central beam themore the resolution and sharpness of the image increases.

Using an axial illuminating beam on an object such as the diatomAmphipleura pellucida, creates diffraction orders over such a largeangle that even the highest numerical aperture objectives availablecannot include all of them. The specimen's markings remain unresolvedand thus invisible.

The use of oblique lighting can result in the inclusion of one or moreorders of wavelets for a specimen which are lost when illuminated byaxial lighting The greater the angle of the oblique light the greaterthe number of orders of wavelets included within the objective apertureand thus the greater the resolving power of the system. In fact, boththe resolution as well as the sharpness of the image can besignificantly increased compared to axial illumination, because theoptimal oblique illumination will place the zero order wavelet near theedge of the objective aperture and thus, the objective can recombinemore orders of diffraction wavelets for any given structural detail.

Accordingly, it is a principal object of the present invention toprovide an illumination system and method for a transmitted lightmicroscope which produces oblique lighting having the maximum anglepossible for the lenses used thereby enhancing the microscope'sresolving power and sharpness of image.

In conjunction with the object stated above is an object of theinvention to utilize the entirety of the beam or beams directed onto thecondenser as illumination sources for the specimen. That is to say, thatthe present invention, unlike so much of the prior art, does not use amask on the condenser or between the condenser and the specimen tocreate an oblique light beam from a small portion of the beam initiallydirected onto the condenser.

It is a further object of the invention to provide for a transmittedlight microscope having a condenser lens, an illumination system whichproduces an oblique light beam which is independently selectivelyadjustable in both the location and angle at which it exits thecondenser lens.

While the use of a single illuminating beam according to the presentinvention achieves results which can surpass the prior art in terms ofresolution, and is within the scope of the invention, the maximumpotential of oblique lighting is achieved in the present invention whena plurality of independent beams are used. Specifically, while a singlebeam system produces enhanced resolution, it does so predominantly alongthe direction of the beam axis (projected onto the specimen plane).Furthermore, at 90 degrees to that axis there is a significant decreasein resolution and sharpness. For example, in order to see the detailedpattern of Amphipleura Pellucida the specimen must be rotated on thestage so that the markings are oriented along the axis of the obliqueilluminating beam. As the specimen is rotated away from that optimalposition, the markings become less distinct and finally disappearaltogether. As the specimen is rotated further, the markings becomevisible again as the orientation approaches 180 degrees. This is aresult of the fact that while a single oblique beam increases resolutionalong an X dimension, it decreases resolution along the perpendicular Ydimension. If, however, two oblique beams illuminate a specimen so thattheir angle of orientation is 90 degrees apart, then the imageresolution and sharpness is increased in both the X and Y dimensions.Enhanced resolution over essentially the entire specimen plane isachieved using multiple oblique illuminating beams radially spaced aboutthe optical axis of the condenser. As a result, very fine structuraldetails such as the markings on Amphipleura Pellucid a can be seenregardless of how the specimen is oriented on the stage.

When multiple beams are used, enhanced resolution is derived not onlyfrom the benefits of oblique illumination but also from the overallincrease in the system's N. A. (numerical aperture) that results frommultiple beams following different oblique paths from the condenser tothe objective. That is, the "working" N.A. of the condenser beam isincreased beyond its normal potential because a highly oblique beam oflight will exit the condenser lens at a greater angle than will a normalaxial beam. The increased exit angle will only be on one side of thecondenser while the exit angle will be deficient on the opposite side ofthe condenser. If however, a second oblique light beam is directed intothe condenser at the opposing angle relative to the first beam, thenboth sides of the condenser will project an exit beam with a greaterangle than would be possible with a single central light beam. Thus,multiple oblique light beams can be directed into a condenser lens atopposing angles relative to the optical axis such that the resultingexit beams will combine to form an overall increase in the aperture ofillumination and thus, an increase in the overall resolution of thesystem. The final resolution of the image is dependent on the N.A. ofthe system. For microscopes using an objective lens along with acondenser lens, the N.A. of the system will be the combination of theN.A. of objective and condenser lenses.

Thus, another object of the invention is to provide an illuminationsystem and method for a transmitted light microscope utilizing aplurality of independent, separate illuminating light beams directedonto a condenser wherein each light beam follows a different obliqueangled exit path to the objective (relative to the objective's opticalaxis).

Another object of the invention is to provide an illumination system fora transmitted light microscope utilizing a plurality of independentseparate illuminating light beams directed onto a condenser wherein theexit path of each light beam from the condenser is independentlyadjustable in both its location and angle. Such a system enjoys, inaddition to the advantages already stated, the advantage of being ableto significantly increase the depth of field without degradation ofresolution.

It is well known that in a conventional illumination system for amicroscope, reducing the condenser aperture to increase depth of fieldand contrast, reduces resolution. A known alternative method forincreasing depth of field is to slightly under focus the condenser lens(keeping the condenser aperture fully open) while closing a field stopiris to increase depth of field. If a single illuminating beam is used,whether it be axial or oblique, then the increase in depth of field willbe accompanied by a decrease in resolution. In the present invention,multiple oblique beams are directed onto the condenser so that even whenthe field lens aperture is reduced to increase depth of field andcontrast, resolution is not degraded. This follows because overallaperture of illumination at the condenser lens, which continues toreceive and transmit light beams from its full aperture, has not beenreduced. Put in another way, the final image is the combination ofmultiple images, each with extended depth of field created by an arrayof pre-apertured oblique illuminating beams, which have an additiveeffect on the overall aperture of illumination.

Yet another object of the invention is to provide means for using doubleoblique lighting in a transmitted light microscope having a condenserlens which produces enhanced resolution and real time 3-D viewing withextended depth of field.

By directing separate independent illuminating light beams onto thecondenser, it is possible in the present invention to manipulate eachbeam independently if desired, such as by interposing complementaryfilters and thereby produce true, real time 3-D viewing. Theinterposition of polarizing filters in the path of one or more beamspermits a variety of effects, such as selective shadow rotation, to beachieved at the same time that enhanced resolution is realized.

Other objects of the present invention will in part be obvious and willin part appear hereafter.

A significant part of the present invention teaches how to realize themaximum potential of oblique illumination by directing two or moreseparate and distinct oblique light beams into the condenser lens in avariety of configurations in order to achieve results which would not bepossible with a single illuminating beam. Some of those configurationswill be illustrated and their advantages discussed. However, there areother possible configurations that will not be specifically discussedbut still fall within the scope of these teachings.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages of the inventionwill be better understood from the following detailed description of thepreferred embodiment of the invention with reference to the drawings inwhich:

FIG. 1A is a schematic diagram of microscope optical elements (includinga condenser lens and an objective lens) wherein the illumination path iscoincident with the axes of the lenses;

FIG. 1B is a schematic diagram of the microscope optical elements ofFIG. 1A wherein the illumination path is parallel to but not coincidentwith a condenser lens and oblique to the objective lens;

FIG. 1C is a schematic diagram of the microscope optical elements ofFIG. 1A wherein the illumination path is non-coincident with and obliqueto the axes of both lenses;

FIG. 1D is a wave diagram illustrating the relative number of orders ofwavelets that can be seen by the objective lens by the illuminationarrangement of FIG. 1A;

FIG. 1E is a wave diagram illustrating the relative number of orders ofwavelets that can be seen by the objective lens by the illuminationarrangement of FIG. 1B;

FIG. 1F is a wave diagram illustrating the relative number of orders ofwavelets that can be seen by the objective lens by the illuminationarrangement of FIG. 1C;

FIG. 2 is an optical schematic illustration of a two beam embodiment ofthe invention;

FIGS. 2A and 2B are plan views illustrating two possible mirrorarrangements for the embodiment of FIG. 2;

FIG. 2C is a plan view illustrating a three mirror configuration;

FIG. 2D is a plan view illustrating a four mirror configuration;

FIG. 2E is a plan view illustrating a six mirror configuration;

FIG. 3 is an isometric, optical schematic illustration of a three beamembodiment of the invention;

FIGS. 3A, 3B and 3C are plan views of the mirrors of FIG. 3 in varyingarrangements;

FIG. 4 is an isometric illustration of a beam shift means of theinvention for mirrors.

FIG. 5 is an optical schematic illustration of the embodiment of FIG. 2with the mirrors replaced by fiber optic guides; and

FIG. 6 is an isometric illustration of a beam shift means which is thesame as that shown in FIG. 4 but with the mirrors replaced by fiberoptic guides.

DESCRIPTION OF PREFERRED EMBODIMENTS

An important aspect of the present invention is best described withreference to FIGS. 1A-1C wherein a light beam path shift means (mirror)11, a condenser lens means 12 having an optical axis 13, an objectivelens means 14 having an optical axis 16, and a specimen support stage 17disposed between the condenser 12 and the objective 14 and defining aspecimen plane 20, are the basic components of a microscope illuminationsystem. The support stage 17 holds a specimen (not shown) to beilluminated by light beam 18 from a light beam source means (not shown).The axis 13 of the condenser 12 and the axis 16 of the objective 14 areshown as being coincident which is the most common arrangement fortransmitted light microscopes. Such axial coincidence is not required bythe present invention, however, which is equally operative in a systemwhere, for example, the condenser is tilted relative to the objective.Although both the condenser means12 and the objective means14 are eachshown diagrammatically as a single lens, it will be understood by thoseskilled in the art that the condenser means and the objective means maybe comprised of multiple elements as well as other optical devices knownin the art.

When as shown in FIG. 1A, the mirror 11 is positioned on the condenseraxis 13 and disposed at a 45 degree angle relative to the initial path19 of beam 18, which is normal to the condenser axis 13, the path 21a ofthe beam after being shifted by the mirror 11 will fall along the axis13.

Unless otherwise stated, lines indicated as representing a beam pathsuch as 19 and 2la, are schematic representations of a beam's axis. Inreality, of course, a beam has an envelope which can be converging,diverging or parallel. An understanding of the present invention is bestfacilitated, however, by following the path of a beam's axis.

As is well known, a beam incident a condenser such as 12 along its axis13 will emerge from the lens along an axial path 22a. For thearrangement of FIG. 1A the beam path 22a will pass through the specimenplane 20 at right angles thereto and include the objective lens 14 alongits axis 16. FIG. 1A represents a typical "bright field" illuminationsystem.

When mirror 11 is laterally displaced from the axis 13 of condenser 12while being maintained at a 45 degree angle, as shown in FIG. 1B, theshifted beam path 2lb remains parallel to condenser axis 13 but islaterally displaced therefrom. The effect of the beam path 2lb enteringcondenser 12 at an off axis location is to create an angle β between theexit beam path 22b and the objective axis 16. However, the exit locationof the beam 22b from the condenser means 12 is not laterally displacedfrom the condenser optical axis 13.

Since the specimen plane 20 is at right angles to the objective axis 16the beam path 22b will be angled or oblique to a specimen in thespecimen plane 20. For the purposes of the present invention, however,the important relationship is the angle β between the exit beam path 22bfrom the condenser 12 and the optical axis 16 of the objective 14. Theadvantages of the present invention do not, for example, accrue from asystem that creates an oblique angle between the specimen plane and theilluminating beam path by tilting the specimen stage while at the sametime allowing the illuminating beam to travel a path that is parallel tothe objective axis. Such an arrangement still produces standard "brightfield" illumination enhanced only by some possible shadowing.

Referring to FIG. 1C, mirror 11 is inclined relative to the path 19 ofincident source beam 18, to be greater than 45 degrees (50 degrees forexample), causing an angle of reflectance for the beam that sets thebeam path 21c to the condenser means 12 at an angle Ω relative to theoptical axis 13 of the condenser means. The effect of beam path 21centering condenser 12 at an angle Ω is to laterally shift the locationof the exit beam path 22c from the center of the condenser lens means 12to some location nearer the periphery. Thus, by changing the angle ofthe mirror 11 relative to the beam path 19, as well as laterallydisplacing it off of axis 13, as shown in FIG. 1C, the beam path shiftmeans 11 is operative not only to control the angle of the exit path 22cbut its location on the condenser 12 as well. Under these circumstances,the angle of oblique illumination is increased (angle μ in FIG. 1C isgreater than angle β in FIG. 1B). Thus, the angle of the exit beam path22c from the condenser 12 is a function of the lateral (radial from axis13) displacement of the incident beam path 21c, while the lateral(radial from axis 13) position of the exit beam path 22c on thecondenser 12 is a function of the angle of the incident beam path 21crelative to the axis 13 of the condenser means 12.

One of the important features of the present invention is themanipulation of the beam path shift means 11 to maximize the angle ofthe exit beam from the condenser within the limits of the aperture ofthe objective. This will depend on the specification of the lenses, suchas the focal length, working distance and numerical aperture.

The maximum oblique angle of the beam path 22c relative to the objectiveaxis 16 within the objective aperture, is achieved by having the beampath exit the condenser at or very near the edge of the condenser lens12. This is achieved by varying the angle of mirror 11 relative to thesource beam path 19 and thereby the angle Ω of the beam path 21c to thecondenser. At the same time, in order to create true oblique lighting,all or a portion of the beam must enter the objective, requiring thatfor a particular objective, the exit beam path from the condenser be ata particular angle as well as location, And, as explained above, theangle μ is varied as a function of the radial location of the mirror 11and thereby the radial location of the entering beam path 21c relativeto the condenser optical axis 13.

One of the advantages that accrues to the present invention is thatcondensers of maximum size can be advantageously used in most systemssince the beam path shift means permits the angle Ω of the exit beampath from the edge of the condenser to be shifted until it includes theobjective. In this way, the best glass can be used and the maximum beampath angle achieved with the result of greatly enhanced resolution.Furthermore, in the present invention, unlike prior art systems, most ofthe optimally angled light beam can enter the objective rather thanmerely just an edge or small portion of the light cone, thereby creatingthe brightest possible image for available light.

While the single beam system described above is capable of greatlyenhancing a microscope's resolution, the improved resolution isprimarily along the direction of the axis of the illuminating beam (asprojected onto the specimen plane), with the resolution along adirection 90 degrees thereto being significantly degraded.

Resolution and sharpness are ultimately dependent upon the number oforders of diffraction wavelets that can be collected and recombined bythe objective lens. FIGS. 1D,1E and 1F illustrate the relative number oforders of wavelets that can be seen by the objective lens under theilluminating conditions shown in FIGS. 1A, 1B and 1C, respectively. InFIG. 1E, which corresponds to the oblique illuminating conditions ofFIG. 1B, the objective lens collects and recombines more orders ofdiffraction wavelets 25 than shown in FIG. 1D which corresponds to theaxial illuminating system of FIG. 1A. However, the increase in the orderof wavelets collected in the X dimension is linked to a decrease in theorder of wavelets collected in the Y dimension. This increase (ordecrease) in resolution relative to the resolution attainable with axialillumination, is proportional to 2 times the cosine of angle Φ, whereangle Φ is the angle of orientation of the specimen (not shown) relativeto the axis of the oblique illumination. Angle Φ ranges from 0 to 90degrees, where 0 degrees is the X dimension (or the axis of obliqueillumination) and 90 degrees is the Y dimension.

In FIG. 1F, which corresponds to the maximum oblique illuminatingconditions, as shown in FIG. 1C, the number of diffraction wavelets 25collected and recombined by the objective is even greater than thenumber attainable with the oblique illuminating conditions shown by FIG.1B and 1E. This results from the fact that the objective lens is viewingthe wave front at such a highly oblique angle that the spacing of thewavelets appears foreshortened and so more wavelets can be seen by theobjective. This additional increase in resolution is proportional to thesine of the angle between the axis of the oblique illuminating beam andthe optical axis 16 of the objective lens means.

Thus, there is an increase in resolution that is related to the amountof lateral displacement of the illuminating beam and there is also anincrease in resolution that is related to the angle of the illuminatingbeam relative to the optical axis. The total increase in resolution isthe combined effect of both of these elements.

One of the outstanding features of the present invention is that theillumination beam shift means (i.e. mirror 11) is located off thecondenser axis thereby permitting a plurality of such beam shift meansto operate within the system simultaneously. Thus, improved resolutionover the entire specimen plane can be achieved by utilizing a pluralityof illuminating beams positioned to have their respective axes atselected angles to one another.

Referring to FIG. 2, a pair of beam path shift means in the form ofmirrors 23 and 24 disposed off the optical axis 13 of condenser 12permit the system to operate with two independent illuminating beams tothe condenser lens means 12. A light beam source means (lamp) 26 directsa light beam 27 along a source beam path 28 that leads to the beam pathshift means 23. Similarly, a light beam source means (lamp) 29 directs alight beam 31 along a source beam path 32 which leads to beam path shiftmeans 24. Mirror 23 shifts the direction of beam path 28 to path 28awhich passes through condenser 12. Mirror 23 is disposed a distanceradially away from the condenser axis 13 and at an angle π relative toits incident light beam 27 which produces the exit beam path 33 fromcondenser 12 to emerge from the edge of the lens at the maximum anglewhich includes the objective 14. Similarly, mirror 24 shifts thedirection of beam path 32 to path 32a which includes condenser 12.Mirror 24 operates in precisely the same way as mirror 23 to produce thedesired exit beam path 34 from condenser 12.

The relationship of the locations of mirrors 23 and 24 relative to axis13 is shown in FIG. 2A, but can be different depending on the resultsdesired. For example, the shift means can be disposed in essentiallyopposing relationship (180 degrees apart) for 3-D viewing purposes asshown in FIG. 2A, or at essentially right angles (90 degrees apart) asshown in FIG. 2B, to achieve the best overall resolution for a two beamsystem. Resolution over the entire specimen plane is improved byincreasing the number of beams. A three beam system as shown in FIG. 2C,where the beam shift means 30 are evenly angularly spaced (120 degreesapart) about axis 13, provides improved resolution over the entirespecimen plane. Increasing the number of beams even further to four(FIG. 2D) or to as many as six (FIG. 2E) will produce even betterresults. Because of the off axis placement of the beam shift means,numerous other arrangements of mirrors and spacing are possible to meetspecific needs.

For purposes of the present invention, the source of light beams 27 and28 (FIG. 2) can be from separate independent light beam sources asshown, or from a light beam source means providing a single light beamwhich is split by beam splitting means (not shown) which are well knownin the art. More important than the source of the light, are themultiple beams 27 and 31 directed along separate, independent paths tothe condenser, and the resultant exit beam paths 33 and 34 which do notfall along the optical axis 16 of the objective lens means 14.

Likewise, while mirrors provide one means of beam shifting, other meansexist, such as prisms, and the fact that all such means are not showndoes not mean that any of them are excluded from the invention. Thepresent invention, in fact, encompasses an arrangement of separate microlight sources 101, as could be provided using fiber optics 102 (see FIG.5); with the beam shift means comprising mechanical or electro-mechanical means 103 (see FIG. 6); for positioning and directing theselight sources. In all cases, the invention is manifest by separate,independent, light beams directed to the condenser means.

Additionally, for the purposes of the present invention, the beam shiftmeans is shown to be adjustable in order to accommodate a large varietyof different objective lenses. However, with a given objectivelens/condenser lens combination, there is no necessity for an adjustablebeam shift means, and a fixed or pre-adjusted beam shift system wouldsuffice. Thus, the present invention includes such fixed systems knownin the art that will direct light beams into a condenser lens at theappropriate location and angle of orientation.

One of the outstanding features of the multiple beam embodiment of thepresent invention is the intensity of light available to illuminate thespecimen at the specimen plane 20. Unlike prior art devices that createangled light beams, the present invention does not require the use ofmasks or other light occluding devices. Thus, the present inventionmakes it possible to increase the light from the light beam source meansfor illumination of the specimen. While the light beam source means hasbeen shown schematically as a light bulb, it will be understood by thoseskilled in the art that the light beam source means may include anysuitable source of light as well as lens means and other optical deviceswell known for the purpose of furnishing object illuminating radiation.

Another important feature of the multiple beam embodiment is that it isable to overcome the anisotropy that is inherent in all obliqueilluminating systems known in the prior art. The anisotropy ofresolution and sharpness has been discussed above. Another effect of theanisotropy associated with prior art systems is the obvious unevenillumination of the image field. That is, one side of the field of viewappears bright while the opposite side appears dark. The introduction inthe present invention of multiple beams makes it possible to produce anevenly illuminated filed of view.

The utilization in the present invention of a plurality of light beamsfollowing different paths to the condenser makes it possible toindividually manipulate those beams for a variety of possible results inaddition to enhanced resolution. For example, referring to FIG. 2, realtime 3-D is achieved by interposing complimentary polarizing filters 36and 37 in beam paths 28 and 32, respectively together with providingsimilar eye piece polarizing filters 38 and 39 in binocular eye piece 41having a pair of viewing lenses 42 and 43. The filters 36 and 37 aredenoted by positive and negative symbols to indicate that they could becomplementary in a variety of different ways known in the art. They maybe plane polarizers oriented with their polarizing axes mutually atright angles. Alternatively, they may be circular polarizers, one of thepair producing left-hand polarization, the other producing right-handpolarization. Yet in another alternative, the filters may becomplementary color filters (such as red and green) of either theabsorption or dichroic type. The eye piece filters 38 and 39 interactwith filters 36 and 37 to selectively limit the light from only one ofthe light sources 26 and 29 so that the image produced by the lightalong beam path 33 does not exit the viewing lens 43, and the imageproduced by the light along beam path 34 does not exit the viewing lens42.

The overlap of the filtered beams which is possible by adjustment ofbeam path shift means 23 and 24 creates real 3-D images and by beingable to independently control the direction of the light paths of thebeams, it becomes possible to control the parallax angles for left andright images, and thereby control the degree of depth perception in thefinal image.

An evenly spaced four beam system (not shown) in which one pair ofadjacent beams provides the illumination for one eyepiece and the otherpair of adjacent beams provides the illumination for the other eyepiece,provides the advantage of overall high resolution inuring to a system oftwo beams at right angles, with 3-D viewing. It follows that a six beamsystem of two pairs of three beams would give even greater resolutionfor 3-D viewing.

The present invention goes far beyond what can be achieved with a singlebeam, real time, 3-D system in which the degree of parallax is fixed,andthere is very little disparity in parallax between the left and rightimages, especially at the center of the image field. In contrast, withthe present invention the left and right images are independentlycontrolled and the degree of parallax between them can be easilyadjusted to match the type of objective being employed and the type ofspecimen being viewed. In addition, there is another and possibly evenmore important advantage with the present invention, which is theability to increase depth of field without attendant loss of resolution.This is a critical prerequisite for producing a sharp 3-D image.

A microscope utilizing the illumination system of the present inventioncan use any of the many light beam manipulation devices known inmicroscopy, such as polarizing filters, aperture stops, collimator, etc.In multiple beam systems of the present invention these devices can beused to provide beams having different characteristics or those havingthe same characteristics.

Since resolution is enhanced by oblique illumination primarily along theaxis (in both directions) of the illuminating light beam, while beingdiminished along the axis 90 degrees thereto, a first orderapproximation of high resolution over the entire specimen plane isachieved using two beams. Adding more beams will further enhance thedistribution of high resolution over the specimen plane. However, littleis gained by using more than 5 or six oblique light beams, radiallyspaced about the optical axis. As can be seen from the previousdiscussion about the anisotropy of resolution associated with a singleoblique beam (FIGS. 1E and 1F), the fall-off in resolution is negligiblewithin 15 degrees or so either side of the axis of each illuminatingbeam (it is proportional to the cosine of that angle).

By way of example for a 3 beam system, referring to FIGS. 3, 3A and 3B,mirror surfaces 45, 46 and 47 are supported on beam shift means 48, 49and 50, respectively. Each mirror surface is disposed in one of thesource beam paths 51, 52 and 53, of light beams 54, 56, and 57,respectively, emanating from light beam sources means 58, 59 and 61. Theshift means 48, 49 and 50 as best seen with reference to FIGS. 3A and3B, are movable along paths 55 that are radial relative to the opticalaxis 13 of the condenser means 12 (see FIGS. 2 and 3). For purposes ofthe present invention, the beam shift means are positioned at locationson their paths 55 that place the beam reflecting mirror surfaces 45, 46and 47 radially outward from the axis 13. As fully described above,varying the location of a mirror (45 for example) along its radial path55 varies the angle of the beam exit path 66 (see FIG. 3) from condenserlens means 12.

Referring to FIG. 3C, "bright field" illumination is available in thepresent system by locating one of the mirror surfaces (47 for example)over the optical axis 13 and in a position in which it creates a beampath that travels along the condenser lens means axis 13. The othermirror surfaces can be deployed to provide oblique lighting at the sametime or disabled (mirrors moved out of range of the condenser means orassociated light beam source means turned off) for standard "brightfield" illumination.

Positioning of the shift means 48, 49 and 50 can also result in "darkfield" illumination. When the radial location of the mirror surfacescreate beam exit paths from the condenser means that are angled to falloutside of the objective aperture, "dark field" illumination is madepossible.

Referring to FIG. 3, each mirror surface 45, 46 and 47 is also angularlytiltable relative to its associated source means beam so as to vary theangle of reflectance of its mirror surface. Thus, by tilting a mirrorsurface the angle of the beam path from the shift means to the condensermeans 12 is varied and in turn the location of the exit beam path fromthe condenser means is varied.

The source means beams 54, 56 and 57 follow source beam paths 51, 52 and53 to the light path shift means 48, 49 and 50 that are generally normalto the axis 13 of condenser means 12 and evenly angularly spaced aboutthe axis 13 of the condenser12 and the axis 16 of the objective lensmeans 14 which axes are shown as being coincident (see FIG. 2). Themirrors 45, 46 and 47 are positionable radially and angularly toestablish the direction of the beam paths 62, 63 and 64 to the condenserlens means 12, and thereby control the location and direction of theexit paths 66, 67 and 68 from the condenser means to the objectivemeans..

The practicalities of size and space between the shift means and thecondenser lens means 12 makes it very difficult to gather all of thelight from the individual beams 54, 56 and 57 and direct it ontocondenser lens 12 at precisely the location and angle necessary toachieve the desired exit paths from condenser lens 12. A large fieldlens 71 (such as a 50 mm f/1.2 camera lens) acts as a pre-condenser lensmeans permitting the gathering of all of the light from the incidentbeams and the accurate direction of those light beams onto the condenserlens 12. The raising or lowering of the field lens 71 relative to thecondenser lens 12 has the effect of sizing the beam on the specimenplane 20 to accommodate low power as well as high power systems.

Furthermore, a field lens aperture (iris stop) 72 can be used to controldepth of field and contrast, provided the condenser means 12 is slightlyunder focused. Prior art systems reduce the condenser lens aperture toincrease depth of field but in doing so reduce resolution due to aconcomitant reduction in the numerical aperture of the light beamexiting the condenser. However, in the multiple beam embodiment of thepresent invention, the condenser aperture 69 remains fully open whilethe field lens aperture 72 can be reduced to increase depth of fieldwithout a concomitant loss in resolution. This is because the apertureof each illuminating beam is reduced while the overall aperture ofillumination that exits the condenser lens is not significantly reduced.The multiple beams illuminate the full condenser aperture and no loss ofresolution is experienced.

The interposition of the field lens 71 and iris stop 72 does notinterfere with the operation of the present invention since adjustmentof the mirror surfaces 45, 46, and 47 continues to control the directionand location of the exit paths of the beams from the condenser lens 12.

Likewise, the interposition in the source beam paths 54, 56 and 57 ofsuch devices as lamp condensers 73, zoom lens 74 (to adjust beam size),and polarizing filters 76 does not interfere with the operation of thepresent invention and in fact highlights one of its major advantages.The use of such light manipulating devices on the light beams eitherseparately (between the light source and the shift means) or together,such as by the field lens 71, the field lens aperture (iris) 72 or apolarizing filter 77 (between the shift means and an eye piece 78), doesnot reduce the system's resolution.

Where light sources 58, 59 and 61 are independent (as opposed to asingle source split by optical means) they can be varied in intensity toadd yet a further investigatory variation.

From the forgoing it is apparent that in order to achieve enhancedresolution the present invention does not limit the use of well knownoptical devices for light manipulation nor does it result in operationat low light levels relative to the light provided by the light sourcemeans. Thus, the illumination system of the present invention enhancesresolution and at the same time makes it possible to create illuminationconditions that can satisfy a wide variety of investigation needs.

A multi- beam system of the present invention enjoys enhanced resolutionboth from an increase in the oblique orientation of the illuminatingbeams relative to the objective lens means optical axis (increase inorders of wavelets recombined) as well as from an increase in theoverall aperture of illumination of the condenser lens due to theadditive effect of the multiple light beams that exit the condenser fromaround its periphery.

When polarizing filters 76 in the source beam paths 54, 56 and 57 fromthe light source means to the beam shift means are complementary,rotation of polarizing filter 77 in the combined beam between theobjective lens means14 and the eyepiece lens 78 permits rotation of theshadow effect of the oblique lighting on the specimen by effectivelyattenuating the illumination from one or two of the beams while lookingat the effects of the other.

The present invention is independent of any particular mechanical orelectrical system for positioning and directing the illuminating beams.This includes systems that may be adjustable or pre-adjusted and fixed,and may utilize mirrors, prisms, fiber optics or other known or unknowndevices. Such mechanical systems can take any number of forms known tothose skilled in the art. By way of example, such a mechanicalarrangement for positioning the mirrors of the shift means is describedwith reference to FIG. 4.

A shift means 80 includes a mirror 81 affixed to a tilt arm 82 which isrotatably connected by hinge 83 to an "L" shaped mount member 84 whichis secured to a car 86 that runs in tracks 87. A cable 88 attached to atab 89 formed on the end of mount member 84 provides the means forpositioning the car 86 on the track 87 and thus the radial position ofthe mirror 81 relative to an optical axis. A pivot arm 91 is pivotallyattached at one of its ends to the tilt arm 82 and at its other end to aslide 92 that runs in a groove 93 in the mount member. A cable 94affixed to a tab 96 formed in the end of slide 92 positions the slide inits groove and in doing so adjusts the tilt of the tilt arm 82 and theangle of the mirror 81. The use of micrometers (not shown) attached tothe ends of the cables 94 and 98 to operate the cables makes it possibleto achieve the degree of precision necessary for the invention.

A number of shift means 80 in the same system can be mechanicallyinter-connected (by means well within the skill of the art) so thattheir positions will be inter-dependent. That is, the movement of oneshift means to a new radial location or the tilting of a mirror to adifferent angular position will cause corresponding movement in theother shift means. This arrangement sets a fixed relationship betweenthe mirrors and makes it possible to easily assure that all of the beamsare substantially identical in their paths through the system other thantheir circumferential position relative to the objective axis 16.

Where it is desired to be able to vary one beam path without disturbingthe others, then the positioning of the shift means is mostadvantageously mechanically independent. In the preferred embodiment ofthe invention the mirror members are selectively mechanicallyinter-connected for unified movement and mechanically unconnected forindependent movement. Such a system is capable of satisfying the needsof a wide variety of microscope uses.

The same mechanical apparatus described above for positioning mirrorscan be used to position fiber optic guides directing light onto thecondenser as shown in FIG. 6.

Once again, the present invention is independent of any particularmechanical system for interconnecting the beam shift means whichmechanical systems can take any number of forms known to those skilledin the art.

The method of the present invention for increasing resolution, sharpnessand depth of field in a transmitted light microscope having a condenserlens means with an optical axis, and an objective lens means with anoptical axis, which is apparent from the forgoing, constitutes the stepsof directing a plurality of independent light beams onto the condenserlens means along paths that are not coincident with the condenser lensmeans optical axis; and fixing the location and direction of the pathsof the light beams to the condenser lens means so that the light beamsthat exit the condenser lens means are directed along paths that includethe objective lens means and are oblique relative the optical axis ofthe objective lens means. Further, the directions of the paths of thelight beams onto the condenser lens means are selected to produce exitpaths from the condenser lens that are at the optimal angle relative tothe optical axis of the objective lens means that includes the objectivelens means.

When the number of beams is two and they are directed along paths thatare in opposition to one another (essentially 180 degrees apart) theyprovide the best illumination for real-time 3-D viewing. When they areat right angles (90 degrees to one an other) they provide the bestoverall resolution using just two beams. When the number of beams isthree or more they are preferably radially positioned and spaced aboutthe optical axis of the condenser lens means for the best overallresolution at the specimen plane.

The invention having been fully described, it is not to be limited tothe details herein set forth, but is of the full scope of the appendedclaims.

What is claimed is:
 1. An illumination system for a transmitted lightmicroscope including a condenser lens means having an optical axis andobjective lens means having an optical axis, comprising incombination:light beam source means located off the optical axis of thecondenser lens means for providing a light beam along a source beam paththat passes through the condenser lens means; and beam path shift meansoperable to shift the source beam path to be non-parallel to the opticalaxis of the condenser lens means.
 2. The invention of claim 1 whereinsaid light beam source means provides a plurality of independent lightbeams along source beam paths that pass through the condenser lensmeans, and said beam path shift means shifts a plurality of source beampaths.
 3. An illumination system for a transmitted light microscopeincluding a condenser lens means having an optical axis and objectivelens means having an optical axis, comprising in combination:light beamsource means for providing a light beam along a source beam path that isnon-coincident with the optical axis of the condenser lens means; andbeam path shift means disposed off the optical axis of the condenserlens means in said source beam path, said beam path shift means operableto shift the path of said source means light beam and redirect it alongan incident path that passes through the condenser lens means, and anexit path from the condenser lens means that passes through theobjective lens means, wherein the path between the condenser lens meansand the objective lens means is oblique to the optical axis of theobjective lens means.
 4. The invention of claim 3 wherein said shiftmeans is operable to variably alter the direction of the path of thelight beam to the condenser lens means over a range that produces exitpaths from the condenser lens means that pass through the objective lensmeans.
 5. The invention of claim 3 wherein said light beam shift meansis operable to variably alter both the location and the angle of theexit beam path from the condenser lens means.
 6. The invention of claim5 wherein said light beam shift means is operable to independently varythe location and the angle of the exit beam path.
 7. The invention ofclaim 6 wherein said light beam shift means comprises a mirror angularlymoveable relative to said source means light beam, and laterally movablerelative to the optical axis of the condenser lens means.
 8. Theinvention of claim 6 wherein the condenser lens means includes apre-condenser lens means.
 9. The invention of claim 8 wherein thepre-condenser lens means further includes a field stop iris.
 10. Anillumination system for a transmitted light microscope including acondenser lens means having an optical axis, and an objective lens meanshaving an optical axis, comprising in combination:light beam sourcemeans for providing a plurality of independent light beams alongdifferent source beam paths which are non-coincident with the opticalaxis of the condenser lens means; and a plurality of light beam shiftmeans each of which is operable to shift the path of a source meanslight beam and redirect it along an incident path that passes throughthe condenser lens means, and an exit path from the condenser lens meansthat passes through the objective lens means, wherein the exit path isoblique to the optical axis of the objective lens means.
 11. Theinvention of claim 10 wherein said light beam shift means are operableto variably alter the direction of the path of their associated lightbeam to the condenser lens means over a range that produces exit pathsfrom the condenser means that passes through the objective lens means.12. The invention of claim 11 wherein said light beam shift means areoperable to variably alter both the locations and angle of the exit beampaths from the condenser.
 13. The invention of claim 11 wherein saidlight beam shift means are independently operable.
 14. The invention ofclaim 11 wherein said light beam shift means are operably interconnectedwhereby the operation of one to vary the path of its associated lightbeam causes a variation in the path of the light beam associated witheach of the other light beam shift means.
 15. The invention of claim 10wherein each of said light beam shift means is fixed to create a singleexit path from said condenser lens means.
 16. The invention of claim 10wherein the condenser lens means includes a pre-condenser lens opticallydisposed in the incident paths of beams from said shift means to thecondenser lens means.
 17. The invention of claim 16 wherein lens isphysically movable relative to said condenser lens means.
 18. Theinvention of claim 16 wherein said pre-condenser lens means furtherincludes an associated field stop iris.
 19. The invention of claim 10wherein the number of light beams and the number of said light beamshift means is two.
 20. The invention of claim 19 wherein theillumination system further includes means for supporting a specimen ata specimen plane between the condenser lens means and the objective lensmeans, and wherein said shift means are operable to establish light beampaths which at the specimen plane are at essentially right angles toeach other.
 21. The invention of claim 19 wherein the illuminationsystem further includes means for supporting a specimen at a specimenplane between the condenser lens means and the objective lens means, andwherein said light beams are essentially 180 degrees apart in opposingrelationship to one another at the location of the specimen plane. 22.The invention of claim 19 further comprising complementary filtersdisposed between said source means and condenser lens means.
 23. Theinvention of claim 22 wherein said filters are plane polarizers.
 24. Theinvention of claim 22 wherein said filters are circular polarizers. 25.The invention of claim 22 wherein said filters are complementary colors.26. The invention of claim 22 wherein the system further includes aneyepiece lens means disposed to receive light from the objective lensmeans and wherein the invention further comprises complementary filtersoptically disposed between the objective lens means and the eyepiecelens means.
 27. The invention of claim 10 wherein the number ofindependent light beams and the number of said light beam shift means isthree.
 28. The invention of claim 27 wherein the illumination systemfurther includes means for supporting a specimen at a specimen planebetween the condenser lens means and the objective lens means andwherein said exit beam paths are evenly angularly spaced from oneanother at the specimen plane.
 29. The invention of claim 27 wherein thesystem further includes an eyepiece lens means disposed to receive lightfrom the objective lens means and wherein the invention furthercomprises polarizing filters optically disposed between said sourcemeans and said condenser lens means and a polarizing filter opticallydisposed between the objective lens means and the eyepiece lens means.30. The invention of claim 27 further comprising a zoom lens opticallydisposed between said source means and said shift means.
 31. Theinvention of claim 27 wherein said beam path shift means comprises threemirrors disposed about the optical axis of the condenser lens means andin the source beam paths, said mirrors movable along paths that areradial relative to the optical axis of the condenser lens means andangular relative to the source beam paths.
 32. The invention of claim 31wherein the movement of one of said mirrors is independent of theothers.
 33. The invention of claim 31 wherein the movement of one ofsaid mirrors causes movement in the others.
 34. The invention of claim27 wherein the different source beam paths include fiber optic means andsaid beam path shift means includes fiber optic position means.
 35. Theinvention of claim 10 wherein said light beam shift means include mirrorsurfaces.
 36. The invention of claim 10 wherein said light beam shiftmeans include fiber optics.
 37. The invention of claim 10 wherein thenumber of independent light beams is four.
 38. The invention of claim 10wherein the number of independent light beams is six.
 39. Anillumination system for a transmitted light microscope including acondenser lens means having an optical axis, and an objective lens meanshaving an optical axis, comprising in combination:light beam sourcemeans for providing a plurality of independent light beams alongdifferent source beam paths to the condenser lens means; a pluralitymirror surfaces optically disposed in source beam paths wherein eachsaid mirror surface is movable radially relative to the optical axis ofthe condenser means and angularly relative to its associated sourcebeam, wherein said mirror surfaces are movable to positions along radialpaths in which they do not intersect the condenser means optical axiswhereby said mirror surfaces are operable to direct source beams ontothe condenser lens means at locations thereon which are determined bythe radial and angular positions of said mirror surfaces.
 40. Theinvention of claim 39 wherein at least one of said mirror surfaces ismovable radially to a position in which said mirror surface intersectsthe optical axis of the condenser lens means.
 41. The invention of claim39 in which the movement of said mirror surfaces is interdependent oneach other.
 42. The invention of claim 39 in which the movement of saidmirror surfaces independent of each other.
 43. The invention of claim 39in which the movement of said mirror surfaces selectably independent ofeach other and interdependent on each other.
 44. The invention of claim39 wherein the range of movement of each mirror surface produces a paththat passes through the objective lens means.
 45. The invention of claim39 wherein the range of movement of each mirror surface produces pathsthat pass through and pass by the objective lens means.
 46. Theinvention of claim 39 wherein the condenser lens means includes apre-condenser lens means.
 47. The invention of claim 46 wherein thepre-condenser lens means further includes a field stop iris.
 48. Amethod of increasing resolution in a transmitted light microscope havinga condenser lens means having an optical axis, and an objective lensmeans having an optical axis, comprising the steps of:directing aplurality of independent light beams onto the condenser lens means alongpaths that are not coincident with the condenser lens means opticalaxis; adjusting the direction of the paths of the light beams to thecondenser lens means until the light beams that exit the condenser lensmeans are directed along paths that pass through the objective lensmeans and are oblique relative the optical axis of the objective lensmeans.
 49. The method of claim 48 wherein the directions of the paths ofthe light beams onto the condenser lens means are adjusted until thelight beams that exit the condenser lens means are directed along pathsthat are at maximum angle relative to the optical axis of the objectivelens means and also pass through the objective lens means.
 50. Themethod of claim 48 wherein the number of beams is two.
 51. The method ofclaim 50 wherein the beams are directed along paths that are essentially90 degrees apart.
 52. The method of claim 50 wherein the beams aredirected along paths that are essentially 180 degrees apart.
 53. Themethod of claim 48 wherein the number of beams is more than two and theyare positioned circumferentially about the optical axis of the condenserlens means.
 54. A method for real-time 3-D imaging with increasedresolution in a transmitted light microscope having an condenser lensmeans having an optical axis, an objective lens means having an opticalaxis, a source means of source light beams and a binocular eyepiecemeans, comprising the steps of:directing two independent light beamsfrom the source means into the condenser lens means along paths that arenot coincident with the condenser lens means optical axis and whichproduce exit beam paths from the condenser lens means which are obliqueto the optical axis of the objective lens means and which pass throughthe objective lens means, disposing complementary filters between thesource means and the condenser lens means; and disposing another set ofcomplimentary filters on the binocular eyepiece means.
 55. The method ofclaim 54 further comprising the step of adjusting the location and angleof the source beam path between the source means and the condenser meansto control parallax.
 56. The method of claim 54 wherein the condenserlens means is further described as including a pre-condenser lens meanslens and iris diaphragm having an iris aperture, further comprising thestep of adjusting the iris aperture to adjust depth of field.
 57. In anillumination system for a transmitted light microscope having acondenser lens means with an optical axis, and an objective lens meanswith an optical axis, the combination comprising:light means forproviding a plurality of independent light beams having paths which areoblique with respect to the optical axis of the condenser lens means,and which pass through both the condenser lens means and the objectivelens means, wherein the paths of the beams between the condenser lensmeans and the objective lens means are oblique to the optical axis ofthe objective lens means.
 58. The invention of claim 57 wherein thenumber of beams is
 3. 59. The invention of claim 57 wherein the numberof beams is
 4. 60. The invention of claim 57 wherein the number of beamsis
 6. 61. An illumination system for a transmitted light microscopehaving a condenser lens means with an optical axis, and an objectivelens means with an optical axis, comprising:light means providing aplurality of independent light beams each having an optical axis whichlies along a path which is non-coincident with the optical axis of thecondenser lens means, which independent light beams overlap at thecondenser lens means and which independent light beams pass through boththe condenser lens means and the objective lens means, wherein the pathsof the optical axes of the independent light beams between the condenserlens means and the objective lens means are oblique to the optical axisof the objective lens means.
 62. The invention of claim 61 wherein thenumber of independent light beams is
 3. 63. The invention of claim 61wherein the number of independent light beams is
 4. 64. The invention ofclaim 61 wherein the number of independent light beams is 6.